1. Field of the Invention
The present invention relates to an elastic wave measurement apparatus capable of measuring, at a high speed, response characteristics of a plurality of elastic wave devices to which a high-frequency signal from a single high-frequency signal generation means is input.
2. Description of the Related Art
In recent years, various types of sensors are developed by using elastic wave devices.
Examples of the elastic wave device include a SAW device configured to excite a surface acoustic wave (hereinafter referred to also as a SAW), and device utilizing a quartz crystal microbalance (hereinafter referred to also as a QCM).
A SAW device configured to excite a surface acoustic wave is provided with a piezoelectric substance and comb-like electrode. The piezoelectric substance is a material in which a strain is caused when an impulse signal or high-frequency burst signal is applied thereto, and crystal, langasite or the like is used as the piezoelectric substance. Examples of the SAW device include, as representatives, a device obtained by forming a piezoelectric substance into a flat shape, and device obtained by forming the piezoelectric substance into a spherical shape, which is a spherical surface acoustic wave device (hereinafter referred to also as a ball SAW device).
The flat SAW device is provided with comb-like electrodes at both ends of the piezoelectric substance. In the flat SAW device, when an input signal is applied to the comb-like electrode at one end thereof, a surface acoustic wave of an ultrasonic wave is produced. Further, when the surface acoustic wave propagates to the comb-like electrode at another end thereof, a voltage is produced in the comb-like electrode, thereby making it possible to acquire an output signal. Here, a frequency of the surface acoustic wave is determined by the elastic constant of the base material, and interval between the comb-like electrodes. Accordingly, such a SAW device is used mainly as a small-sized RF filter.
Further, there is a type of flat SAW device in which a comb-like electrode is arranged in the vicinity of a center of the propagation path, and reflectors are provided at positions on the right and left opposed to each other. According to this type of flat SAW device, the surface acoustic wave excited at the comb-like electrode is reflected by the reflectors, and goes back and forth past the comb-like electrode. Further, when the surface acoustic wave passes the comb-like electrode, an output signal is acquired. Here, while the surface acoustic wave repeats the round trip, i.e., while the wave makes the first round trip, second round trip, . . . , if a substance adheres on the propagation path, the velocity of the surface acoustic wave changes. Thus, by measuring a change in the velocity of the surface acoustic wave, application of the SAW device to various types of sensors is enabled.
In the ball SAW device, the piezoelectric substance is formed into a spherical shape, and a comb-like electrode is arranged on the propagation path of the ultrasonic wave. When an impulse signal or high-frequency burst signal is applied to the comb-like electrode of the ball SAW device, a surface acoustic wave is produced on the surface of the piezoelectric substance. The surface acoustic wave goes around the spherical piezoelectric substance, and returns to the original comb-like electrode. At the comb-like electrode, a high-frequency RF signal can be acquired. Further, the surface acoustic wave on the surface of the piezoelectric substance continues to go around the substance, and makes multiple circuits such as 10, 20 . . . 100 circuits, . . . Owing to the above characteristics, it is possible for the ball SAW device to take a longer propagation distance than the flat SAW device in spite of the size thereof smaller than the flat SAW device. Further, the ball SAW device includes no reflector, and hence the loss in the ball SAW device is small. Accordingly, it is possible to utilize the ball SAW device not only as a high-frequency filter, but also as a high-sensitivity sensor.
In the ball SAW device, for example, when the surface acoustic wave goes around the propagation path of the piezoelectric substance, if a substance adheres on the propagation path, the velocity or intensity of the surface acoustic wave is lowered by the mass-loading effect, or, depending on the adhering substance, the surface on the propagation path is hardened, whereby the velocity of the surface acoustic wave is increased, or the intensity thereof is changed. Further, even if a substance does not adheres to the surface of the propagation path, by a change in the atmosphere around the propagation path, the lost amount of energy of the surface acoustic wave changes, and hence the velocity or intensity thereof changes. Accordingly, by observing a change in the velocity or intensity of the surface acoustic wave, it is possible to detect whether or not a substance has adhered on the propagation path. Here, although a drop in velocity or intensity for one circuit is very small, when the number of circuits is increased, the change is also increased. As a result of this, in the ball SAW device, it is possible to make the propagation distance of the surface acoustic wave long, and hence it is possible to detect the adhering substance at high sensitivity (see, for example, Jpn. Pat Appln. KOKAI Publication No. 2005-333457).
Further, by forming, in advance, a film into which a specific substance is absorbed (hereinafter referred to as a sensitive film) on the propagation path of the surface acoustic wave, it becomes possible to specify the adhering substance.
Furthermore, by increasing the number of types of sensitive films, it becomes possible to simultaneously specify a plurality of types of substances, this being applicable to a gas sensor, odor sensor, and the like. However, the sensitive films are formed for each ball SAW device, and hence a plurality of ball SAW devices are required to examine a plurality of types of substances.
It should be noted that measuring the intensity of an output signal from a spherical surface acoustic wave device is nothing but observing an attenuation factor of the surface acoustic wave in the process in which the surface acoustic wave makes a circuit. Further, it is possible to obtain the attenuation factor from a change in the intensity of the surface acoustic wave concomitant with the propagation of the surface acoustic wave.
It should be noted that the term ‘phase’ used for a high-frequency signal generally means, when a predetermined time is defined, a temporal position of the corresponding signal at the defined time. Phase measurement in the output measurement of the spherical surface acoustic wave device generally refers to measuring a temporal position (phase) of a high-frequency output signal from the spherical surface acoustic wave device at the time at which a predetermined time has elapsed from the time at which the surface acoustic wave is excited by using Fourier analysis, quadrature detection, or wavelet transformation. Further, measuring the propagation (circling) velocity of the surface acoustic wave directly from the above measurement, or, obtaining the time at which, for example, the spherical surface acoustic wave device has finished outputting a predetermined number of times (the time at which the spherical surface acoustic wave device has finished circling a predetermined number of times), and obtaining a temporal distance from the circling start time at the obtained time is also called “measuring the phase”. By measuring the phase in the manner described above, information on the propagation (circling) velocity of the surface acoustic wave is acquired in some cases.